Velocity measuring SONAR systems are typically used for submersible shipboard applications for which continuous Global Positioning System (GPS) information is not available. There are two basic types of velocity measuring SONAR systems—Doppler and correlation, each of which has advantages and disadvantages. Doppler SONAR is useful for shallow to moderate ocean bottom depths for which transmission losses associated with its off angle transmissions and receptions are tolerable. But those transmission losses generally preclude the use of Doppler SONAR for deep depths. Correlation SONAR involves transmission and reception normal to the ocean bottom and is thus not fraught with severe transmission loss issues even for deep ocean bottom depths. However, the performance of correlation SONAR systems degrades for elevated ships speed and shallow bottom depths because of physical limitations in the size of the hydrophone array.
Correlation SONAR processing to determine the velocity of a ship involves correlation of two echo signals received within a hydrophone array from pulses transmitted at different times. Correlation is made between the initial transmission echo received on a “reference” hydrophone in the array and the later transmission echo received on each of the array hydrophones. An estimate of distance traveled by the ship is made by identifying the hydrophone which provides the maximum correlation with the initial transmission echo received on the reference hydrophone. This spatial separation is equal to the total distance traveled during two time periods (the time between the two transmissions and time between the two echoes). The time between the two transmissions is called the correlation time (denoted as τ) and is chosen so that the physical location of the correlation maximum is consistent with the size of the hydrophone array. Since the measured distance is over two periods, distance is converted to velocity by dividing by twice the correlation time to provide a “pulse pair velocity estimate”. FIG. 1 depicts the correlation process 110, a correlation time of bottom return echoes 120, and a typical correlation SONAR hydrophone array geometry 130.
Highly accurate correlation SONAR systems require use of ocean bottom echoes vice water volume reverberation information which is contaminated by ocean currents. Furthermore, because of the rapidly increasing attenuation of sound in water with frequency, deep depth ground referenced correlation SONAR systems must generally use transmit frequencies which do not exceed 20 khz. Such low frequencies adversely impact accuracy as illustrated by the theoretical random error model for correlation SONAR horizontal velocity measurements:
                              σ          H                =                              K            1                    *                                    1              N                                *                      1            τ                    *                      λ                          θ              BW                                *                      (                          1              +                              (                                                      P                    N                                                        P                    S                                                  )                                      )                                              (        1        )            wherein
K1 = a constant
N = number of independent samples of data in bottom return echo
λ = transmit frequency length
τ = correlation time
PN = ambient noise power
Ps = echo signal power
ΘBW = composite transmit/receive beam width and λ = c/f0 in which
c = speed of sound in water
f0 = transmit frequency
Deep depth correlation SONAR systems can be designed to provide good accuracy despite the transmit frequency constraint by employing a sufficiently large hydrophone array (to obtain a long correlation time) and a relatively wide transmit/receive beam width. A wide beam pattern benefits accuracy directly via parameter ΘBW. It also improves accuracy by increasing N. It can be shown that:N=K2*fs*D*∫θ∫φSf(θ,φ)B(θ,φ)dθdφ  (2)where:
K2 = a constant
D = depth below keel
fs = number of independent samples of data per second
Sf(θ, φ) = ocean bottom echo scattering function
B(θ, φ) = composite transmit/receive beam pattern function wherein θ and φ are the beam angles off the main response axis in polar coordinates
It can further be shown that the number of independent samples of data per second, fs, is inversely proportional to 1/PW, where PW is the transmit pulse width. Sf, the ocean bottom echo scattering function, is a complex function of bottom type involving bottom roughness, bottom slope, and transmit frequency which establish the ocean backscattering characteristic that affects echo duration.
A deep depth correlation SONAR system can be expected to provide degraded accuracy in shallow water because (a) the correlation time is constrained by short signal roundtrip time, and (b) echo duration can be expected to provide a small number of samples (parameter N in Equation No. 1). In an embodiment, shallow water is a depth of water wherein the achievable correlation time is less than the desired correlation time (See FIG. 3). Ground referenced shallow water correlation SONAR systems achieve high accuracy by using relatively high transmit frequencies (e.g., 75-300 khz). Such SONAR systems can provide operation for all ocean bottom depths by transitioning to the much less accurate water referenced mode for deep bottoms.
A correlation SONAR system transmits a series of pulses vertically towards the ocean bottom, opens a receive window at a time prior to the expected location of the first bottom return echo, keeps the window open long enough to receive the entire pulse train, and provides an additional time period for data processing during which neither transmissions nor receptions take place. This completes a “SONAR cycle” and successive cycles follow. The events within a SONAR cycle are illustrated in FIG. 2.
Typical SONAR systems provide excellent velocity accuracy over wide range of bottom depths, but exhibit degraded performance over shallow depths because of the aforementioned constraints on τ and N. A desired correlation time is computed (denoted τd) such that, for the example hydrophone array shown in FIG. 1, the correlation peak will occur at nominally 3 hydrophone spacings from the reference hydrophone in the direction of maximum ship's velocity (fore-aft or athwartships):
                              τ          d                =                              (                          3              *              H                        )                                (                          2              *                              max                ⁡                                  (                                                            abs                      ⁡                                              (                                                  V                          FA                                                )                                                              ,                                          abs                      ⁡                                              (                                                  V                          ATW                                                )                                                                              )                                                      )                                              No        .                                  ⁢        3            wherein
VFA = ships fore-aft velocity
VATW = ships athwartships velocity
h = adjacent hydrophone separation
The SONAR system selects receive pulse pairs for velocity estimation whose separation most closely approximates τd. This approach maximizes use of the hydrophone array thereby providing greatest accuracy if the transmit burst duration is at least τd seconds in duration. Accuracy can be considered “degraded” when the desired correlation time (τd) cannot be obtained due to insufficient transmit burst duration. The limit for degraded operation occurs when τd equals the burst duration. This leads to a hyperbolic curve (Speed*Depth=constant) because τd is inversely proportional to Speed, where Speed=max abs(VFA), abs(VATW)) from Equation No. 3) and the burst duration is directly proportional to depth, D. Velocity error steadily increases in the “degraded region” (τ<τd) in which the error grows according to τd/τ. FIG. 3 depicts the operating regions for a deep depth ground referenced correlation SONAR system. The error level steadily increases as depth and speed decrease within the degraded region. This model has been shown to provide a good estimation of shallow depth error levels. For deep depths, however, there is no dramatic error growth as Speed approaches zero.
The approaches described in this background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this background section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this background section.